[0001] The present invention relates to a novel method for the manufacture of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid of formula (I):
a retinoid local anti-acne agent, used in pharmaceutical compositions, in particular
for the treatment of certain types of acne.
[0002] The compound of formula (I) is described in particular in patent
EP 0 199 636. Patent
EP 0 199 636 describes the preparation of this compound according to the following
Scheme 1:
[0003] In this method, 2-(1-adamantyl)-4-bromoanisole is converted, in a first step (a),
to its organomagnesium compound, and then to its organozinc compound by the action
of zinc chloride (ZnCl
2), and is then coupled with methyl 6-bromonaphthoate. This reaction is catalyzed by
a transition metal (palladium or nickel) or one of its complexes with various phosphines.
The synthesis of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid of formula (I),
as described in patent
EP 0 199 636 (Scheme 1), is therefore carried out in three steps with a yield of 63%, from 2-adamantyl-4-bromoanisole.
[0004] One of the disadvantages of this reaction is the formation of impurities. One of
these impurities results from the reaction of the organozinc compound generated
"in situ" with 2-adamantyl-4-bromoanisole to give 3,3'-di(1-adamantyl)-4,4'-dimethoxy-1,1'-biphenyl
having the following structure:
[0005] Another impurity also forms via the transfer of the zinc compound of 2-(1-adamantyl)-4-bromoanisole
to methyl 6-bromonaphthoate and the reaction of this novel zinc compound with methyl
6-bromonaphthoate to give the dimerization product having the structure:
[0006] This product leads, after saponification (step (b)) and acidification (step (c)),
to the following impurity:
[0007] A third impurity forms during the hydrolysis of the reaction medium. Indeed, during
this hydrolysis, the unreacted organozinc compound of 2-(1-adamantyl)-4-bromoanisole
generates the impurity having the following structure:
[0008] At an industrial stage, these impurities are difficult to remove from the finished
product and most often require reprocessing by recrystallization.
[0009] Furthermore, some catalysts such as [1,2-bis(diphenylphosphino)ethane]nickel chloride
(NiCl
2(dppe)) must be prepared separately, adding a step to this method.
[0010] During the coupling reaction in step (a), the acid functional group is protected
in methyl ester form. This acid functional group should be regenerated. Thus, in a
second step (b), methyl 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoate is saponified
by treating with a base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH)
under reflux in an alcohol such as methanol.
[0011] By acidifying the reaction medium with hydrochloric acid, 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid is obtained in a third step (c). It should also be noted that in this method,
the methyl ester of 6-bromo-naphthoic acid should be prepared in one step from the
corresponding acid. It can therefore be seen that the prior art method is complex
and is not entirely satisfactory.
[0012] In this context, one of the objects of the present invention is to provide a method
for preparing 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid, which is a lot
simpler and more economical. The method according to the invention should be more
suited to an industrial production, more particularly in terms of cost and compliance
with Good Manufacturing Practices. One of the objectives of the invention is to provide
a novel method for preparing compound (I) which does not require in the final stage
deprotection of the acid functional group, avoiding the formation of the impurities
mentioned above and making it possible to reduce the number of synthesis steps.
[0013] In this context, the subject of the present invention is a method for preparing 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid of formula (I):
by a single-step Suzuki reaction between 3-adamantyl-4-methoxyphenylboronic acid of
formula (II):
and 6-bromo-2-naphthoic acid of formula (III):
[0014] The subject of the invention is also the use of the compound (II), on the one hand,
and the use of the compound (III), on the other hand, for the preparation of the compound
of formula (I).
[0015] According to the method of the invention, it is therefore possible to couple in a
single step the (1-adamantyl)phenyl part and the naphthyl part of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid.
[0016] The preparation of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid, according
to the method of the invention, is illustrated in
Scheme 2 below:
[0017] The preparation of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid (I) is carried
out by the Suzuki reaction between 3-adamantyl-4-methoxyphenylboronic acid (II) (prepared
in particular according to a method similar to that described in patent applications
WO 02/072009 A2 and
WO 03/011808 A1) and the commercially available 6-bromo-2-naphthoic acid (III). The preparation of
6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid of formula (I) as described by
this novel method according to Scheme 2 is carried out in two steps from 2-(1-adamantyl)-4-bromoanisole
(IV) (prepared for example according to
EP 0 199 636), with a yield which is much higher than that obtained with the prior art method.
As shown by the example which follows, the yield of the method according to the invention
may be of the order of 95% or higher.
[0018] In the context of the invention, the Suzuki reaction is carried out by coupling between
compounds (II) and (III) in the presence of a palladium catalyst and a base, in a
polar solvent, followed by an acid treatment.
[0019] Typically, the Suzuki reaction may be carried out in the presence of a palladium
catalyst such as palladium(II) acetate, tetrakis(triphenylphosphine)-palladium(0),
palladium on activated charcoal or dichloro[1,1'-bis(diphenylphosphino)ferrocene]-palladium(II),
in an aprotic polar solvent (for example acetonitrile, N,N-dimethylformamide, dimethoxyethane
or tetrahydrofuran) or a protic polar solvent (for example n-propanol, i-propanol)
or a mixture of these solvents with water. The volume of solvent used will be between
7 and 13 times the quantity of 6-bromo-2-naphthoic acid (III) used and the volume
of water used will be between 7 and 13 times the quantity of 6-bromo-2-naphthoic acid
(III) used.
[0020] Advantageously, the palladium catalyst may contain a ligand chosen from: a triphenylphosphine,
a tri-o-tolylphosphine, a tri-m-tolylphosphine or a tri-p-tolylphosphine. The catalysts
particularly preferred are palladium(II) acetate and palladium on carbon which make
it possible to obtain a particularly fast reaction kinetics. Palladium(II) acetate
may be advantageously used in combination with a 2-(dicyclohexylphosphino)-biphenyl
type ligand (
J.P. Wolfe et al., J. Am. Chem. Soc., 1999, 121, 9550-9561).
[0021] These catalysts may also be encapsulated, like for example the Pd EnCat™ type catalysts.
The reaction is generally carried out in the presence of an inorganic base such as
potassium carbonate, sodium carbonate, caesium carbonate, sodium hydroxide or potassium
hydroxide or in the presence of a tertiary amine such as triethylamine or diisopropylethylamine.
The particularly preferred bases are potassium carbonate, potassium hydroxide and
diisopropylethylamine.
[0022] The Suzuki reaction is preferably carried out under an inert atmosphere, for example
under an argon or nitrogen atmosphere. The reaction mixture is advantageously heated
at a temperature in the range from 60° to 110°C, for 30 minutes to 24 hours. A treatment
in an acidic medium, for example in the presence of HCl, is carried out. It will be
noted that, according to the conditions used in Examples 1 and 2, the kinetics of
the reaction is very rapid and is complete within two hours. Persons skilled in the
art will be able to modify these conditions, in particular by applying the variants
of the Suzuki reaction which are described in the literature (
N. Miyaura & A. Suzuki, Chem. Rev., 1995, 95, 2457-2483;
A. Suzuki, J. Organomet. Chem., 1999, 576, 147-168). The method according to the invention is therefore simple and economical and makes
it possible to directly obtain compound (I) with a high yield, close to quantitative.
[0023] This new method also makes it possible to obtain 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid with a high degree of purity in which the impurities obtained in the prior art
method are completely absent. The following preparations illustrate the method of
the invention.
Example 1:
a) - Preparation of 3-adamantyl-4-methoxyphenylboronic acid (II):
[0024] 100 g (0.311 mol) of 2-(1-adamantyl)-4-bromoanisole (IV) and 500 ml of THF are introduced,
under nitrogen, into a 2 L three-necked reactor. The reaction medium is cooled to
-75°C. 137 ml (0.342 mol) of a 2.5 M nBuLi solution are added. After stirring for
1 h at -70°C, 80 ml (0.342 mol) of triisopropyl borate are added. After returning
to room temperature, the reaction mixture is hydrolysed with 1 litre of 1.2 N HCl.
The aqueous phase is extracted with ethyl acetate and the combined organic phases
are washed with 1 litre of saturated NaCl, and then with 1 litre of water. The organic
phases are dried over sodium sulphate and the solvents are evaporated. 88.37 g of
a white solid are obtained, which solid is reimpasted in 440 mL of heptane. After
filtration, the precipitate obtained is rinsed with heptane, and then dried under
reduced pressure at 35°C until a constant weight is obtained. 84.4 g of 3-adamantyl-4-methoxyphenylboronic
acid are obtained in the form of a white solid - (yield = 94.8%; m.p. = 263°C).
[0025] 1H NMR (CDCl3): δ: 1.77 (s; 6H); 2.10 (m; 3H); 2.20 (s; 6H); 3.91 (s; 3H); 7.00 (d; 1H; J
1=8.0 Hz); 8.05 (dxd; 1H; J
2=1.5 Hz and J
1=8.0 Hz); 8.15 (d; 1H, J
2=1.5 Hz)
b) - Preparation of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid (I):
[0026] 20 mL of tetrahydrofuran (12 vol), 2 g (7 mmol) of 3-adamantyl-4-methoxyphenylboronic
acid (II), 1.65 g (6.6 mmol) of 6-bromo-2-naphthoic acid (III) and 20 mL of a 2 M
aqueous potassium carbonate solution are introduced into a round-bottomed flask equipped
with stirring and under a nitrogen stream. 15 mg (1%) of palladium acetate and 46
mg (2%) of 2-(dicyclohexyl-phosphino)biphenyl are then introduced. The medium is heated
under reflux for 2 hours. Kinetic monitoring by HPLC indicates that the % of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid formed is 94% after one hour and 98% after 2 h.
[0027] After returning to room temperature, the catalyst is filtered on a cartridge, and
then slowly poured over 30 ml of a 1 N aqueous hydrochloric acid solution.
[0028] The medium is kept stirring for one hour. The precipitate is filtered, washed with
water and then dried under reduced pressure. 2.68 g of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid are obtained in the form of a white solid whose purity, determined by HPLC, is
99.9% (yield = 94.8%; m.p. = 321°C) .
Example 2:
Preparation of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid (I):
[0030] 80 g (0.319 mol) of 6-bromo-2-naphthoic acid, 95.7 g (0.335 mol, 1.05 eq) of 3-adamantyl-4-methoxyphenyl-boronic
acid, 0.8 g of 5% palladium on carbon (50% wet, Degussa type E105CA/W) and 800 ml
of tetrahydrofuran (10 vol) are introduced into a 4 litre reactor. The medium is heated
to 55°C. 85 g (1.05 mol, 3.3 eq) of potassium hydroxide at 85% are dissolved in 240
ml of water (3 vol).
[0031] The solution obtained is poured over the reaction medium. The addition is exothermic.
The reaction medium reaches the reflux temperature. The reflux is maintained for about
2 hours.
[0032] The reaction medium is filtered at about 35-40°C on a cartridge and rinsed with 400
ml of a THF/water mixture (1/1).
[0033] The medium is cooled to 20°C and 100 ml of HCl at 35% in 600 ml of water are added.
6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid precipitates. It is filtered
and washed with 4 litres of water. The pH of the washings is about 6-7. The product
is dried under vacuum at 100°C for 24 hours.
[0034] 131 g of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid are obtained (crude
yield = 99%).
[0035] This crude material is dissolved in 15 to 22 volumes of THF under reflux. After filtration
in the hot state, 15 to 22 volumes of heptane are added and the medium is cooled to
about 5°C for 1 to 2 hours.
The 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid is filtered on sintered glass
and it is rinsed with 1 to 2 volumes of heptane.
[0036] 108 g of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid are obtained in the
form of a white solid whose purity, determined by HPLC, is 99.9% (yield = 82%; m.p.
= 320-322°C).
Example 3:
Preparation of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic acid (I):
[0037] 20 ml (12 vol) of tetrahydrofuran, 2 g (7 mmol) of 3-adamantyl-4-methoxyphenylboronic
acid (II), 1.65 g (6.6 mmol) of 6-bromo-2-naphthoic acid (III) and 20 mL of a 2 M
aqueous potassium carbonate solution are introduced into a round-bottomed flask equipped
with stirring and under a nitrogen stream. 0.7 g (5%) of 10% palladium on carbon (50%
wet; Heraeus type K-0218) is then introduced.
[0038] The medium is heated under reflux for 8 hours. The catalyst is filtered on a cartridge,
and then slowly poured over 30 ml of a 1 N aqueous hydrochloric acid solution.
[0039] The medium is kept stirring for one hour. The precipitate is filtered, washed with
water and then dried under reduced pressure. 2.06 g of 6-[3-(1-adamantyl)-4-methoxyphenyl]-2-naphthoic
acid are obtained in the form of a white solid whose purity, determined by HPLC, is
99.9% (yield = 79%; m.p. = 321°C).